1. FIELD OF INVENTION
This invention relates to a method of forming a hollow article, more particularly to a new method for forming a hollow article, such as a nut blank, which method comprises fewer steps than the prior art method by using a die of new design.
2. DESCRIPTION OF THE RELATED ART
It has been an intention of every inventor to reduce the steps of making a certain article so as to cut down the cost of manufacturing and simultaneously increase the amount of production. So far, the cold extrusion method is widely used to form a hollow article, such as a nut blank.
FIGS. 1 to 4 show a conventional method of forming a nut blank by cold extrusion. The method comprises: (a) cutting a predetermined length of stock (A3) from a long cylindrical stock (A2) by a shear means (A1); (b) conveying said predetermined length of stock (A3) to a cavity confined by two die elements, one of which being stationary and the other one being movable between a closed position in which the stock is securely clamped and an open position in which the stock is free for feeding and being punched in said die (FIGS. 2 and 3 respectively illustrate the predetermined length of stock (A3) being punched by two rods); (c) moving a punch (A5) towards the predetermined length of stock (A3) into the cavity of the die and extruding a certain portion of the predetermined length of stock (A6) from the cavity of the die, thereby obtaining a nut having a blind bore therethrough and simultaneously ejecting a scrap from the die.
In the above-mentioned method, the ejected scrap of the stock is wasted, resulting in the increase of the cost of production.
There already exist a method of forming a nut blank without scrap entitled as U.S. Pat. No. 3,267,500. FIGS. 5(A) to 5(E) illustrate the steps of making a nut blank having a blind bore according to said U.S. Patent. As illustrated, the method includes the steps comprising: (1) moving a free end of a cylindrical stock (B1) deeper into an open die (B2) having a cavity including a first section with polygonal internal wall surfaces (B11) and a second section having a first circle in cross section with a diameter substantially equal to the diameter of the cylindrical stock (B1) fed into the open die (B2) by a flange of a sleeve member (B3) while a punch (B4) moves towards the free end of the cylindrical stock (B1); (2) extruding a certain amount of the cylindrical stock (B1) towards the opening of the open die (B2) from between an exterior of the punch (B4) and an internal wall of the open die (B2), piercing a blind bore (B12) within the cylindrical stock (B1) by the punch (B4) while the sleeve member (B3) moves away from the die; (3) moving a shear means with a receiving space (B5) aligned with the cavity of the open die; (4) conveying of a predetermined length of the extruded section of the cylindrical stock into the receiving space (B5) of the shear means; (5) inserting a stock gauge (B6) of the shear means into the blind bore (B12) of the predetermined length of the extruded section in order to hold it in a position; (6) cutting off the predetermined length of the extruded section (B11) while the shear means moves away from the open die (B2).
The above-mentioned method, though it can produce a nut blank without scrap and thus can cut down the cost of manufacturing, still has a few remaining setbacks:
(1) This method includes, first of all, a step of pushing the extruded section of the stock fully back into the cavity of the open die by the sleeve member. Then the sleeve member retracts back to its initial position before the punch moves towards the stock. In order to achieve the pushing and the retracting operation, the apparatus is installed with a more complicated machinery than necessary.
(2) The first section of the cavity of the die that has polygonal internal wall surfaces, is constructed longer than the desired length of a nut blank to be produced in order to obtain said desired length of nut blank. To construct a die with such a cavity costs more than the cost of construction of a die with a shorter cavity of such type.
(3) A clearance is formed between the exterior of the stock, having a circle in cross section, and the polygonal internal surfaces of the cavity of the die. This clearance can not provide full stability of the stock during the punching operation.